Generally, if you cannot cancel a mistaken CoS in 1s or less, it's kind of fruitless. Cobra Shot will trigger the GCD regardless of whether you finish the shot or not, so in order for it to be "worth it" you need to be casting the nuke at 1s. Even then, you're trading .7 seconds to get your nuke off earlier for the damage of a cobra shot. Unless you're consistently delaying your nukes and end up missing a significant portion of them over the course of the fight because you're not stopcasting and casting tons of mistaken Cobras, the benefit is negligible, and possibly even negative. And if you're casting that many mistaken Cobra Shots, you need to practice your rotation more than relying on stopcasting.

Stopcasting twice is 1) to stop the autoshot, and 2) to stop the currently casting shot. Unfortunately, if you do just one, you'll cancel your autoshot (as opposed to the casting shot), which is silly. It's also doubly important for CC.